Examples of floating decks of the type described may be found described in U.S. Pat. No. 3,910,452 issued October 7, 1975 as well as in co-pending Canadian patent application Ser. No. 253,504 filed May 27, 1976. These decks are platforms made of inter-connected modular units of predetermined shape which are made of a closed cell foam plastic, such as polyurethane, so that they float on the fluids stored in the tank. The modular units are sealingly connected to one another and the border of the platform wipes the inner walls of the storage tank to reduce the loss by evaporation of the stored fuel or fluid chemicals. The assembled platform is usually covered with one layer of fireproof material, such as asbestos, and a layer of expanded metal to provide rigidity and to dissipate static electricity. With these types of modular units, certain problems are often encountered, one of which is liquid absorption by the polyurethane thus resulting in poor sealing between the units, corrosion of the metal layer and, even in some cases, sinking of the entire deck. Another problem is that liquid absorption in the polyurethane greatly reduces the rigidity of the platform.